1. Field of the Invention
The present invention relates to a revolution speed detecting apparatus and, more particularly, to a wheel speed detecting apparatus.
2. Related Background Art
A conventional revolution speed detector is described in Japanese Patent Application Laid-Open No. 9-281125. This device is constructed to extract a signal including angular frequency information according to revolution speed from an alternating-current (ac) signal outputted from a revolution speed sensor. Specifically, through differentiation of the ac signal such as a sine or cosine wave or the like, the angular frequency information becomes included in the amplitude component of signal, and thus it can be extracted.
However, because the amplitude of the revolution speed sensor varies according to the revolution speed, the component of angular velocity information included in the amplitude component becomes reduced relative to noise in a low revolution speed range, so that precise detection of revolution speed is not allowed in the low speed range.
The present invention has been accomplished in view of such a problem and an object of the invention is to provide a revolution speed detecting apparatus capable of detecting the revolution speed with improved detection accuracy.
In order to solve the problem, the present invention provides a revolution speed detecting apparatus for extracting a signal including angular frequency information corresponding to revolution speed from an ac signal outputted from a revolution speed sensor, wherein the amplitude of the ac signal is substantially constant and the ac signal is differentiated so as to permit the angular frequency information to be extracted by determining a ratio thereof.
According to the present apparatus, when the ac signal such as the sine or cosine wave or the like is differentiated, the angular frequency information becomes included in the amplitude component of signal. Through a plurality of differential operations the amplitude including the angular frequency information varies, but the time-dependent term comes to be equal. Since the amplitude of the ac signal outputted from the revolution speed sensor is substantially constant herein, the angular frequency information can be extracted by determining a ratio thereof. Since the apparatus is so constructed that the amplitude of the ac signal is substantially constant and that the above operation is carried out, the present apparatus is able to detect the revolution speed with improved detection accuracy even in such an extremely low speed range as the signal accuracy was null because of too small amplitude output, at least, before.
When the ac signal is a sine wave including the noise component and/or harmonic contents, the waveform of the signal is not a precise sine wave, but it is an approximate sine wave. In the present apparatus, the ac signal is preferably an approximate sine or cosine wave.
Since the present invention can restrain use of an integrator, which was indispensable for the operation heretofore, though not excluding it, the invention is able to decrease the time constant of the signal, so as to permit a stop judgment with high accuracy and an improvement in the detection accuracy of revolution speed to a stop.
In a preferred embodiment, the revolution speed sensor is located near a rotor to be detected, the rotor being comprised of an electroconductive or magnetic body and the circumference of the rotor being comprised of a plurality of portions arranged at predetermined intervals, and the revolution speed sensor comprises a coil for outputting the ac signal according to variation in eddy current or in magnetic permeability of the magnetic body appearing in the portions when the portions go into the vicinity of a magnetic field thereof with rotation.
Particularly, when the revolution speed sensor is a wheel speed sensor, the revolution speed of a wheel as a rotor to be detected can be detected with good responsivity and with accuracy even in the extremely low speed range.